There exist various techniques for improving the contrast of a liquid crystal display apparatus. The following examples are disclosed in Patent Literatures 1 through 7.
Patent Literature 1 discloses a technique of improving a contrast ratio by appropriately setting the content and the specific surface area of a yellow pigment in a color filter. According to this technique, pigment molecules of the color filter scatter and depolarize polarized light. This makes it possible to remedy a problem of a decrease in the contrast ratio of a liquid crystal display apparatus. The technique disclosed in Patent Literature 1 reveals that the contrast ratio of a liquid crystal display apparatus improves from 280 to 420.
Patent Literature 2 discloses a technique of improving a contrast ratio by increasing the transmittance and the polarization degree of a polarization plate. The technique disclosed in Patent Literature 2 reveals that the contrast ratio of a liquid crystal display apparatus improves from 200 to 250.
Patent Literatures 3 and 4 disclose a technique for improving contrast in a guest-host type which exploits a light-absorbing property which a dichroic pigment has.
Patent Literature 3 describes a method for improving contrast by a structure in which a quarter-wave plate is sandwiched between two guest-host liquid crystal cells. Patent Literature 3 discloses that no polarization plate is used.
Patent Literature 4 discloses a liquid crystal display element in which a dichroic pigment is mixed with a liquid crystal used in a dispersed liquid crystal type. Patent Literature 4 discloses a contrast ratio of 101.
The techniques disclosed in Patent Literatures 3 and 4, however, show lower contrast than those of the other Patent Literatures. To further improve the contrast, it is necessary to take various measures: an improvement in the light-absorbing property of the dichroic pigment, an increase in the content of pigment, and an increase in the thickness of the guest-host liquid crystal cell(s). All these measures, however, would lead to new technical problems, such as a poor reliability and a poor response property.
Each of Patent Literatures 5 and 6 discloses a method for improving contrast by use of an optical compensation type in which a liquid crystal panel and a liquid crystal display panel are provided between a pair of polarization plates. The liquid crystal panel performs optical compensation.
In Patent Literature 5, retardation is adjusted in an STN type by using a display cell and a liquid crystal cell for optical compensation. This improves a contrast ratio from 14 to 35.
In Patent Literature 6, a contrast ratio is improved from 8 to 100 by disposing a liquid crystal cell for optical compensation. The liquid crystal cell compensates for wavelength dependence of a liquid crystal display cell of, for example, a TN type, which wavelength dependence occurs while the liquid crystal display cell is displaying black.
Although the techniques disclosed in the Patent Literatures achieve a 1.2 to 10 fold or even greater increase in contrast ratio, the absolute value of the contrast ratio is merely in a range of about 35 to 420.
Another contrast enhancing technique is disclosed in Patent Literature 7, for example. The Patent Literature 7 teaches a complex liquid crystal display apparatus in which two liquid crystal panels are combined whose polarization plates form crossed Nicols. Patent Literature 7 discloses that the combined two panels increase the contrast ratio up to three to four digits. Note that each of the combined two panels, if used alone, merely showed a contrast ratio of 100.
Citation List
Patent Literature 1
Japanese Unexamined Patent Application Publication, Tokukai, No. 2001-188120 A (Publication Date: Jul. 10, 2001)
Patent Literature 2
Japanese Unexamined Patent Application Publication, Tokukai, No. 2002-90536 A (Publication Date: Mar. 27, 2002)
Patent Literature 3
Japanese Unexamined Patent Application Publication, Tokukaisho, No. 63-25629 A (Publication Date: Feb. 3, 1988)
Patent Literature 4
Japanese Unexamined Patent Application Publication, Tokukaihei, No. 5-2194 A (Publication Date: Jan. 8, 1993)
Patent Literature 5
Japanese Unexamined Patent Application Publication, Tokukaisho, No. 64-49021 A (Publication Date: Feb. 23, 1989)
Patent Literature 6
Japanese Unexamined Patent Application Publication, Tokukaihei, No. 2-23 A (Publication Date: Jan. 5, 1990)
Patent Literature 7
Japanese Unexamined Patent Application Publication, Tokukaihei, No. 5-88197 A (Publication Date: Apr. 9, 1993)